For the realisation of numerous industrial, but primarily thermal power station processes it is necessary to extract heat from the process at the ambient temperature level usually via the condensation of the steam-state operating medium of these processes. The traditional solutions involve exceptionally intensive use of water (evaporative or once-through cooling), which, due to environmental protection considerations or the lack of the required amount of water, may cause problems in numerous cases. In order to overcome this various well known and tried dry cooling systems were developed.
The most wide-spread dry cooling system is the so-called direct dry cooling. In this cooling method, if it serves power plant cycles, the water vapour, expanded in a steam turbine subjected to a vacuum, exits from the turbine through a steam pipe with a large diameter, then through an upper distribution chamber it goes into a so-called steam-air heat exchanger. The steam flowing in the fin tubes of the heat exchanger gradually condenses to the effect of the cooling air flowing on the external, finned side of the heat exchanger. As the condensation and heat extraction is realised directly without a transmitting medium, this is called direct dry cooling. Naturally safe and controllable direct cooling by air that can be technically implemented is a much more complex process than this. The process in dry cooling takes place in a decidedly wider temperature range as compared to common water cooling following the significant temperature fluctuations taking place during the year in ambient air temperature. This means that on the steam side significantly varying condenser pressure, in other words turbine back pressure will be created. Taking into consideration these varying temperature and pressure conditions from the point of view of economy it is necessary to select and operate the equipment optimally, as well as to ensure its operational reliability.
The best known and tried direct cooling by air realises the above requirements by breaking down the condensing process into two easily separable phases. In accordance with this the steam-air heat exchanger consists of two parts, the so-called condenser part and the secondary condenser, which is called an aftercooler or dephlegmator in the specialist literature.
The steam exits the steam distribution pipes, then goes through the distribution chambers of the condenser part to the finned heat exchanger tubes. The coolant air flows on the external, finned side at right angles to the longitudinal axis of the pipes, in other words perpendicular to the flow direction of the steam. The condenser may consist of multi-tubes in the direction of the air, but also of a single, extended tube. Due to the cooling effect of the air the steam gradually condenses in the tubes. The condensate goes in the same direction as the steam in a downwards direction due to gravity partially flowing on the internal wall of the tube, partially with the flowing steam to the condensate collection and steam transmission chamber positioned at the bottom end of the pipes. From here the condensate goes from the individual heat exchanger bundles to the condensate pipe. The remaining uncondensed steam (30-15 percent of the initial amount) and the unwanted, non-condensing gases present in the steam pass into a further heat exchanger section, the so-called aftercooler or dephlegmator part.
Significant differences in the degree of condensation and, with this, the concentration of non-condensing gases develop in certain pipe sections both with respect to time and space. Changes over time may be caused by a change in the temperature of the external air, a change in the steam-side loading and the airflow rate. Changes with respect to space are determined by the positioning of the heat exchanger tubes. Significant differences can develop between individual tubes in the plane perpendicular to the direction of cooling airflow due to the uneven steam or air distribution. Further unevenness is displayed in the direction of the airflow, as the cooling air gradually warms up and so is able to condense an increasingly smaller amount of steam. This effect does not only occur in the case of multiple-tube condensers in the flow direction, but also in the case of single row condenser tubes that are stretched out in the airflow direction (although to a lesser degree). The non-condensing gases can become concentrated in certain sections of the heat exchanger, so-called air-plugs can develop, terminating the flow of steam and so removing the tube section of the given heat exchanger from effective cooling. Besides this performance drop, in temperature conditions under freezing, the freezing up of the heat exchanger and significant operation breakdowns can be caused. These problems of direct cooling by air are known of in the related technical journals. (e.g. Kroger, D. G., Air Cooled Heat Exchangers and Cooling Towers, section 8, part 8.2., TECPRESS, 1998).
The problem caused by uneven condensation is reduced by the most widely used direct air cooled system by inserting a heat exchanger section called a dephlegmator, which essentially carries out an aftercooling function. As compared to that justified by the design in general a significantly greater amount of steam is fed from the condenser section to the dephlegmator part due to endeavours to overcome the unevenness. The dephlegmator section uses a similar heat exchanger type to that used in the condensation section, with the significant difference that the input of the steam does not take place from above but from a lower distribution chamber, from which the steam flows upwards in the heat exchanger tubes, in the mean time the condensate flows in the opposite direction to the effect of gravity to the lower steam distribution and condensate collection chamber. The circumstances causing unevenness presented in the case of the condensation section also appear here. One typical problem of this section may derive from steam side overloading, which may hold up the condensate flowing downwards due to the effect of gravity setting up a water plug and so taking out the remaining section of the tube from the operation of the heat exchanger. Over and above this drop in performance this can cause other operation problems, including freezing up problems in cold weather. In accordance with this the dephlegmator section needs to be significantly overdimensioned. A study by Goldschagg, H. B. analyses the problems of one of the most modern direct air cooled systems in existence (Lessons learned from the world's largest force draft direct cooling condenser, paper presented at the EPRI Int. Symp. on Improved Technology for Fossil Power Plants, Washington, March 1993.).
The unwanted, non-condensing gases present in the steam, consisting mainly of air have to be pumped out of the space under vacuum. The pumping work is reduced if the suction takes place in a place where the ratio of the gases in the steam-gas mixture is the greatest. The steam arriving in the upper chamber of the dephlegmator at this point contains ten-fifty percent non-condensing gas, so this steam-gas mixture is suitable for the known pumping out using ejectors. Due to the low steam flow rate in the dephlegmator section a relatively low heat transfer coefficient can be attained. This is made significantly worse by the convective heat transfer which receives an increasing role instead of condensation due to the increasing partial pressure of the non-condensing gases. Besides the heat transfer coefficient a further drop in performance is caused by the reducing steam saturation vapour pressure and temperature due to the increasing partial pressure of the non-condensable gases, and, due to this, the increasingly smaller logarithmic temperature difference. The increasing “undercooling” may be a further source of possible freezing up. This risk is discussed by the analysis in the January 1994 issue of the publication POWER (Swanekamp, R: Profit from latest experience with air-cooled condensers).
A further phenomenon occurring in direct cooling by air during condensation is the drop in pressure of the steam (or steam-gas mixture) flowing in the heat exchanger tubes of the condenser and dephlegmator, which also, naturally, depends on the length of the flow route. This loss of pressure due to friction also reduces the logarithmic temperature difference, which acts as the driving force from the point of view of heat transfer, between the cooling medium (air) and the cooled medium (steam). At the same time due to the large specific volume in the case of a direct air condenser of a given size and reducing external air temperature a status may come about when due to the increasing flow losses the reduction of the temperature of the cooling air does not result in the further improvement of cooling performance (so-called choking). The tube length of the heat exchanger sections of condensers and dephlegmators in the case of average or greater power plant cooling is 10 meters for both, in other words the total tube length is doubled by the dephlegmator section.
The lack of uniformity in both the condenser and the dephlegmator, operation reliability problems and controlling difficulties essentially derive from the fact of the so-called direct condensation itself. The condensation occurring inside the tubes, in the whole of the cooling system, in an extended space sets the amount of steam and steam—non-condensing gas mixture and vice versa, the obstacles reducing, or even blocking the flow reduce or stop the condensation. The lack of forced circulation on the condensing medium side makes the control of the process difficult, and interventions can only take place on the outer side of the heat exchanger, on the cooling air side. This explains why direct air cooled condensers have only been constructed with fans till now. Here the forced circulation of the cooling air gives at least the possibility of regulating the airflow. In the case of natural draught direct condensers on both medium sides the flow is “natural”, in other words the flow is caused by the process itself, and so the process is nearly uncontrollable—this explains why natural draught direct air cooled systems have never been constructed.
Other direct air cooled systems also exist in which the dephlegmator section is not positioned in a separate heat exchanger bundle, but one of the tubes falling in the flow direction of the air is set up as a dephlegmator, or in a so-called “quasi-single tube” system a part in the one tube separated by a wall serves as a dephlegmator. In these cases the imbalance between the individual tubes increases further, and it becomes even more difficult to control the whole process than those presented earlier using separate condenser-dephlegmator heat exchanger bundles. All this does not change that despite the known and operable direct air cooling technical solutions there is a need for a condensation part and following that a so-called dephlegmator section (which is actually a similar direct steam-air heat exchanger in which the condensation process continues).
It can be determined that the most inefficient, in other words the relatively speaking most expensive part of direct cooling by air is the dephlegmator, which, at the same time, is required for reasons of acceptable operation reliability and controllability.
A mention still needs to be made of endeavours that increase the air cooling performance of air cooling, mainly the peak performance by spraying the cooling surface of the finned air cooling tubes with water, or by establishing a continuous film of water on them. Such is presented in the previously referred to Swanekamp publication (POWER, June 1994).